Generally, thermoplastic resins have lower specific gravity than glasses and metals, and have good physical properties such as moldability and impact resistance. Recently, plastic products have rapidly replaced conventional glass or metal products and have been widely used in electrical and electronic goods, as large, light-weight, and economical products are becoming a widespread trend. Accordingly, as good appearance and function of the plastic products have become important when they are used in housings of electrical and electronic goods, there is a need for plastic materials which can provide exterior scratch resistance, impact resistance, and flame retardancy.
Conventionally, a hard coating method has been used to improve the scratch resistance of plastic products. The hard coating method includes coating a surface of an injection-molded resin with an organic-inorganic hybrid material and curing the organic-inorganic hybrid material on the surface of the resin using heat or ultra violet light. However, this method requires an additional coating process which can increase processing time and manufacturing costs and may also cause environmental problems. Recently, with increased interest in protecting the environment and in reducing manufacturing costs, there is a need for a non-coated resin. Thus the hard coating method may not be used but the resin still should have good scratch resistance. Also, it is important to develop a resin with good scratch resistance for the housing manufacturing industry.
One proposed method for improving both scratch resistance and flame retardancy is to alloy polycarbonate (PC) resin and a methacrylate resin, such as polymethylmethacrylate (PMMA).
Polycarbonate resin has good mechanical strength, transparency, thermal stability, self-extinguishing property, and dimensional stability and has been widely used in electrical and electronic products and parts of automobiles. Further, the polycarbonate resin can obtain flame retardancy even with a small amount of flame retardant, compared to a conventional polymer, since its chemical structure allows good flame retardancy. However, good scratch resistance cannot be obtained only with the polycarbonate resin since it has a low pencil hardness of B or so.
On the other hand, although polymethylmethacrylate resin has good scratch resistance, that is, it has a high pencil hardness of 3H-4H, it still has a disadvantage in that it is hard to obtain flame retardancy with a conventional flame retardant.
Accordingly, blending a PC resin and a PMMA resin has been suggested to improve both scratch resistance and flame retardancy. However, although the PC resin and the PMMA resin are melt-kneaded at a high temperature, they divide into separate phases due to their lack of compatibility. It is also very difficult for an alloy of PC and PMMA to be used in housings of electrical and electronic products. Due to the different refractive indices of the PC resin and the PMMA resin, that is, 1.59 and 1.49, respectively, the alloy of PC resin and PMMA resin may scatter light which makes it difficult to provide color with high chroma, and a melt joining line is clearly shown during extrusion.
Korean Patent Publication Laid-open No. 2004-79118 discloses a method of lowering the molecular weight of polycarbonate during a kneading process using metal stearic acid ester in order to improve the compatibility between a polycarbonate resin and a methacrylate resin. However, the method has a disadvantage in that a blend of the polycarbonate and the methacrylate resin has significantly low mechanical properties.